A chap writing in uk.railway a few years back had this detailed explanation:
...The swing link is the 'pendulum' which supports the transverse bolster of the coach's secondary suspension from the bogie frame. In general there are four links, two on each side although some French designs used only two. On the BT10 bogie under the
HST▸ coaches the airbags at each end of the bolster support a transverse 'spring plank' into which the pivot pin of the coach is inserted. The spring plank is located by radius arms in the case of the BT10 to the bolster so there is no relative motion between the two except vertically and in roll. The swing links then permit the coach body to move sideways with respect to the bogie frame under control of the horizontal damper. The longer the swing links the lower the natural frequency of the 'pendulum'.
This design was adopted as at the time there was no experience of using air bags as swing links. Sometime later the bogie under the Class 313 electric units used airbags which combined the vertical spring and horizontal flexibility functions and this has since become standard.
In the initial HST design the swing links were as long as possible and the ends hung below the bolster. At each end of the swing links are longitudinal 'knife edges' to permit the link to swing with low friction on the bogie frame and bolster and which also transfer the body weight from the bolster (at the lower end of the link) to the bogie frame at the top of the link. To permit the suspension to be set up accurately these 'knife edges' could be adjusted by large nuts and lock nuts on screw threads cut into the ends of the link. The
protruding threaded end at the bottom of the link could get quite close to the third rail in Southern territory under critical conditions of worn tyres, body roll and third rail positional tolerances. As a result to permit HSTs to operate on the Southern the swing link was modified to get the lower end away from the third rail by raising the lower knife edge and omitting (or shortening) the lower threaded portion. Although this allowed HSTs to run in third rail areas it meant that the coaches lost the silky lateral motion they had as new as the natural lateral resonance frequency was raised.
Archived here:
http://newsgroups.derkeiler.com/Archive/Uk/uk.railway/2009-06/msg00365.htmlWhat has also confused matters is that there are explanations online that misleadingly concentrate on the safety straps - the 'wire rope assemblies' shown in
bnm's pictures on both the
SSL▸ and
LSL‡ bogies. Some LSL bogies have a steel 'stirrup' rather than a wire rope - this can be seen in the line drawing above, and from what I've seen this is also quite near the third rail. Indeed one online resource reads as though the wire rope and steel stirrup are the swing links, and they are the defining features of the two types - although it is now obvious that wire ropes are fitted to both SSL and LSL types, although I believe that the 'stirrups' are only present on some LSL bogies.
The explanation above suggests that original LSL bogies have a better ride, though I wonder if it is noticeable to anyone other than an engineer. I expect
BR▸ 's decision to only modify a certain proportion of the HST sets was financially driven.
Paul
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